The most important functions of the epidermis that is the outermost layer of the skin are protective functions to protect the body from various external stimuli (physical and chemical stimuli, including chemical substances, pollutants, dry environments and UV rays) and to prevent the excessive evaporation of water in the body through the skin. Such protective functions can be maintained only when the stratum corneum (horny layer) consisting of keratinocytes is normally formed. The stratum corneum that is the outermost layer of the epidermis is formed from keratinocytes and consists of differentiated keratinocytes and lipids surrounding the keratinocyts (J. Invest. Dermatol. 1983; 80: 44-49). Keratinocytes are characteristic cells formed through a process in which basal cells continuously proliferating in the lowest layer of the epidermis undergo a stepwise change in the shape and function thereof and rise to the surface of the skin. After the passage of a given period of time, dead keratinocytes are removed from the skin and replaced with new keratinocytes, and such a repeated process is called the “differentiation or keratinization of epidermal cells”. During the keratinization process, keratinocytes produce intracellular lipids (ceramide, cholesterol and fatty acid) with natural moisturizing factors (NMF) while forming the stratum corneum, such that the stratum corneum has firmness and flexibility to function as a skin barrier.
However, the stratum corneum easily loses its function due to living habitual factors, such as excessive washing or bathing, environmental factors, such as dry atmospheres or pollutants, and endogenous diseases, such as atopic skins or senile skins. In fact, persons suffering from dry skin symptoms due to various factors, and various disorders caused by the dry skin symptoms, have recently gradually increased. Thus, in order to maintain the skin water content at a suitable level, many studies focused on supplying water from an external source or on preventing the loss of water from the body have been conducted, and in fact, various kinds of moisturizers having water-holding capacity have been developed and mainly used in the cosmetic field.
However, as environmental factors harmful to the human body gradually increase and the aged population increases rapidly, persons having skin conditions, in which the turnover rate of the stratum corneum decreases, the lipid synthesizing ability of keratinocytes decreases, or the normal division, growth and differentiation of cells in the epidermis do not become smooth, so that the amount of moisturizing factors and lipids in the stratum corneum is reduced, and thus the normal function of the stratum corneum is not maintained, that is, the skin barrier function is impaired, are gradually increasing.
Such abnormal division and differentiation of epidermal cells result in various skin diseases, including dry skin (xerosis), atopy and psoriasis. When moisturizers having a water-holding function in themselves are used, such skin diseases can be slightly alleviated, but it is difficult to expect the fundamental treatment of the skin diseases.